1. Field of the Invention
This invention relates to modified polyisocyanates and to the plastics derived therefrom. More specifically, it relates to urea group-containing polyisocyanates and flexible polyurethane foams based on these products.
2. Description of the Prior Art
Urethane, allophanate biuret, urea, carbodiimide, and/or isocyanurate group-containing organic polyisocyanates, which are generally dissolved in unmodified polyisocyanates, of the same type, are the subject of numerous patents and patent applications. For example, urethane group-containing polyisocyanates are found in DE application No. 25 13 793 (GB Pat. No. 1,450,660) and DE application No 25 13 796 (GB Pat. No. 1,444,192); allophanate group-containing polyisocyanates, in GB Pat. No. 994,890; biuret group-containing polyisocyanates, in DE application No. 1,215,365 (U.S. Pat. No. 3,441,588); urea group-containing polyisocyanates, in DE application No. 1,008,484 (GB Pat. No. 791,852); carbodiimide group-containing polyisocyanates, in EP application No. 57,862; and isocyanurate group-containing polyisocyanates, in DE Pat. No. 2,616,416 (GB Pat. No. 1,571,933).
In this way, polyisocyanates which are solid at room temperature can be liquified to improve processibility or, for liquid polyisocyanates having a high vapor pressure, volatility can be reduced by increasing the molecular weight.
As specified in European Pat. Nos. 4617, 4618, and 4879, flexible polyurethane foams having high load-bearing and impact absorption ability, or low flammability, can be prepared from diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates mixtures (subsequently referred to as crude MDI) having high contents of diphenylmethane diisocyanate (subsequently referred to as MDI), or urethane group-containing crude MDI. The disadvantage with this process is that the resistance of such flexible polyurethane foams to aging at elevated temperatures and high humidity, for example that required for upholstery cushioning material in the construction of automobiles, frequently is not adequately met.